Heat treated steel alloys have been utilized to die cast molten aluminum and other alloys into solid shapes for many years. The higher melting temperature of steel, about twice that of aluminum, allows it to cool and solidify the aluminum when they come into contact. It may also be used to solidify other lower melting temperature metals or alloys containing large amount of lead, zinc, magnesium, copper, tin, etc.
This same characteristic has been effectively utilized for direct casting of molten aluminum materials (e.g., molten aluminum) to strip form using water-cooled, roll caster shells made of steel alloy (or alloy steel). The molten aluminum is made to flow between two rotating roll caster shells mounted on water-cooled cores. The caster shells extract heat, so that the temperature of the aluminum falls below its melting point and becomes slightly solidified. In this way, a solid aluminum strip can be formed by pulling out from the opposite side.
Because the surfaces of the roll caster shells experience a thermal cycle or a drastic change in temperature from near room temperature to the temperature of molten aluminum (about 1250° F.) as they revolve, numerous small cracks eventually form on the shell surfaces. These cracks (or “heat checks”) are formed by a mechanism known as thermal fatigue, as discussed in U.S. Pat. No. 4,409,027, which is incorporated by reference herein in its entirety. The small surface cracks referred to as “heat checks” or “craze cracks” by those in the industry, eventually grow to the point where they can create marks on the surface of the aluminum strip that even subsequent cold rolling cannot remove the crack patterns from the aluminum strip. At that point, the casting operation must be shut down, the rolls have to be removed and the shell surfaces have to be machined down to their original crack-free condition. Casting may then begin again. However, crack formation, recurs after repeated use. Because of the thermal cycling driving their formation, they can never be fully eliminated. Consequently, the metallurgical design of the shell steel or particularly its alloy is based on retarding the onset of these defects in order to lengthen the service life of the roll shells.
To design an alloy for desired performance and production life, several material properties are considered and controlled. A more detailed description of these material properties and their effect on performance and production life of roll shell alloys is set forth in U.S. Pat. No. 4,409,027, and in U.S. Pat. No. 5,599,497, all of which are incorporated by reference herein in their entirety. As discussed in U.S. Pat. Nos. 4,409,027 and 5,599,497, the desired material properties have low thermal expansion coefficient, high thermal conductivity, high elevated temperature yield strength, high elevated temperature ductility, and a low modulus of elasticity. The most easily controlled property in steel alloys of this type is to increase the elevated temperature yield strength by the additional of selected alloying elements. However, attempts to provide higher elevated temperature yield strength usually result in lower thermal conductivity of the alloyed roll shells. As such, although some improvements in service life were obtained for roll shells by elevating the temperature yield strength, the resulting reduction of thermal conductivity of the roll shells (which reduces aluminum strip production yield) may offset any gain achieved by the elevation of the temperature yield strength. That is, in a conventional roll shell, high temperature yield strength results in low thermal conductivity.
Accordingly, there is a need for roll shells with alloys that can provide the roll shells with not only high temperature yield strength, but also with high thermal conductivity.